Before, amd64 and 386 stored the flags in p->from.scale
and arm and ppc64 stored the flags in p->reg.
Both caused special cases in printing and in handling of the
addresses.

To avoid possible conflicts with the real meaning of p->from
and to avoid storing a non-register value in a reg field,
use from3 to hold a TYPE_CONST value giving the flags.

There is still a special case for printing, because the flags
are specified without a $, and normally a TYPE_CONST prints
with a $. But that's much less special than what came before.

This allows us to remove the textflag and settextflag methods
from LinkArch. They are no longer architecture-specific.

Change-Id: I931da8e1ecd92e127cd9aa44ef5a73c42e730110
Reviewed-on: https://go-review.googlesource.com/3572Reviewed-by: Austin Clements <austin@google.com>